Personally I love punBB since its so basic and lightweight, I think its under 300kb to install. Ive used phpBB and vBulletin but vB is expensive but pretty good. I did like invision powerboards too on Pixel2Life. Vanilla I had problems with but its pretty flat and simple.

A year ago I would have gone with the consensus answer and said VB but Invision came out with their latest release and has blown by VB.

I also lost a WHOLE LOT of respect for VBulletin when they came out with their blog product. They should be paying people to use it not the other way around. It was the most hideous blog blog product I have ever seen in my life and releasing that as a paid solution should have been insulting to all VB users.